The present invention generally relates to head up display (HUD) systems and, more particularly, to backlighting for such systems for improved illumination in both daylight and night operation.
Head up displays (HUD) are known and are currently used in many military and commercial aircraft. HUDs are also finding application in automobiles. Conventionally a HUD unit is mounted in the automobile to project an image in front of the driver. Adapting HUD units for use in automobiles necessitates making the units rugged, reliable, cost effective, and functional in a wide range of environmental conditions.
One problem with HUDs in automobiles is maintaining satisfactory image illumination over a wide range of lighting conditions, for example, daylight and nighttime lighting conditions. Daylight conditions generally require a bright or high intensity light source so that the projected image is visible in the bright light environment. Conversely, nighttime or low light conditions require low brightness or dim light to project an appropriately illuminated image that provides a safe and comfortable viewing experience for an operator.
Conventional HUD units have attempted to address this problem by using a high brightness light source and simply dimming the light source as needed. However, proper illumination in all light conditions may require dimming ratios of 1000:1 or more. This is difficult to achieve since high brightness light sources typically do not function well over such a wide lighting range. Several problems may occur as a high brightness light source is dimmed to low illumination. Such problems include reducing the lifetime of the light source, color shift (e.g. changes from white to yellow), and uneven illumination or bright spots.
Accordingly, it is therefore desirable to provide for a HUD that has improved operation in both daytime and nighttime conditions. It is also desirable to provide a HUD system design that preserves the lifetime of the light source. It is further desirable to improve the light color for a HUD operating in low light conditions. Yet, it is further desirable to reduce the effect of uneven lighting or bright spots generated in low light conditions.
The present invention provides for a head up display (HUD) having multiple light sources of varying intensities for improving the projected image under a wide range of ambient lighting conditions. A high brightness light source illuminates a HUD active matrix liquid crystal display (AMLCD) during bright or daylight conditions. A light panel is interposed between the high brightness light source and the AMLCD and diffuses the high brightness light before it illuminates the rear of the AMLCD. A low brightness light source illuminates an edge of the light panel during low light and nighttime conditions. The light panel diffuses the low brightness light and reflects the light out of the front of the light panel to illuminate the rear of the AMLCD. Light from both the high and low brightness light sources are mixed by the light panel and illuminate the AMLCD during intermediate ambient lighting conditions. The light sources are continuously dimmed or brightened responsive to changes in ambient lighting conditions to provide an improved projected image. An optional light mixer, adjacent to the high brightness light source, also mixes the high brightness light thereby further reducing uneven lighting and light spots.
The high brightness light source may be any of a variety of bright light sources including, but not limited to, halogen lamps, bright LEDs, arc lamps, and fluorescent lamps. Preferably, the high brightness light source is sufficiently bright to project a good quality image on the windshield or combiner during bright daylight conditions and is capable of being dimmed for use in intermediate lighting conditions.
The low brightness light source also illuminates the rear of the AMLCD via the light panel. However, the low brightness light source is preferably located to the side of the light panel and illuminates the edge of the panel. The light panel reflects and diffuses the light from the low brightness light source and illuminates the rear of the AMLCD. The low brightness light source is any of a variety of light sources including, but not limited to, fluorescent lamps and LEDs. The low brightness light source preferably generates good quality light in low light conditions and is capable of being intensified to generate satisfactory light in intermediate lighting conditions.
According to more specific aspects of the present invention, the light panel is light transmissive and allows light from the high brightness light source to pass through and illuminate the rear of the AMLCD. Similarly, the panel reflects light from the low brightness source to the rear of the AMLCD. The light panel is preferably made of acrylic material, includes reflective edges, and at least one textured surface. The reflective edges reflect light back into the panel thereby increasing brightness. The textured surface, preferably on the front (nearest the AMLCD), diffuses the light from both sources for even light distribution.
The invention also includes a light mixer for mixing the light from the high brightness light source. High brightness light sources often produce uneven light or light spots as they are dimmed to low light levels. The preferred embodiment uses a square light mixer that reduces the effects of the uneven light and/or the light spots.
The intensity of the light sources is controlled responsive to ambient light conditions. A light sensor senses ambient light conditions and communicates a light signal representative thereof to a controller. The controller analyzes the light signal and controls the intensity of the light sources for optimum projected image illumination. Brightness of the lights is also adjustable by an operator. The operating ranges of the light sources overlap for improved lighting quality and so that transition between the light sources is smooth and transparent to a viewer. The light sources are controlled as follows. The high brightness light source is active for daylight conditions. As ambient light conditions dim, the high brightness light source is also dimmed. The low brightness light source is activated during intermediate ambient light conditions. Light from the low brightness light source mixes with light from the high brightness light source to reduce color shift (i.e. color temperature light) problems. As ambient light conditions dim further, the high brightness light source is turned off. In low light conditions only the low brightness light source is active, and is dimmed or brightened responsive to changes in ambient light conditions.
The design of the present invention advantageously reduces the problems of the prior art to provide an improved daylight and low light HUD. First, mixing light from multiple sources solves the color temperature light problem. Second, light from both sources mix to reduce uneven lighting or light spots. Third, the lifetime of the high brightness light source is preserved since it need not be dimmed to very low brightness levels.
It is envisioned that multiple high brightness and low brightness light sources may be used. Further, light sources of various light intensities may be used. For example, a high brightness light source may be combined with an intermediate brightness light source and a low brightness light source.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.